Multilayer, dielectric color filters have applications in numerous areas, e.g., radiometry, colorimetry, spectroscopy, laser communications, etc. One application which has emerged recently is in the area of color separation for solid-state color cameras. In this application color filters are used for encoding color images captured by solid-state cameras having single, or multiple, image sensor chips.
In a multi-chip color camera, images are split optically (using beamsplitters) into multiple images (typically three). Each image is directed through a color filter and projected onto one of several, identical, sensor chips. A different filter is used with each sensor chip to split the image into component colors; either (or combination of) primary colors, Red-Green-Blue (RGB), or secondary colors, Cyan-Magenta-Yellow (CMY). Because the filters are discrete (i.e., not an integral part of the image sensor chips) and considerably larger than the optically sensitive elements (i.e., pixels) in the sensor chips, a number of different type color filters, including multilayer dielectric color filters, may be used. Encoding is accomplished, in general, one color per chip. Each chip captures the same image, at the same time, with the same number of pixels. Pixel-to-pixel registration is established between chips and provides the component colors of the image at each pixel location.
In a single-chip color camera, images are captured by either single or multiple exposure(s). Images captured by multiple exposures are separated into component colors by use of multiple filters. One color is captured per filter per exposure. Multilayer, dielectric color filters, unpatterned, are typically the filter type of choice in this application although a number of different filter types may be used. Images captured by single exposure are separated into several (typically three), non-overlapping, one-dimensional or two-dimensional patterns and encoded one component color per pattern. Only one component color of the image is captured per pixel location. Color filters must be patterned to cover individual (or linear arrays of) pixels to permit suitable sampling of spatial color. Often, the process used to pattern the filters imposes constraints on the type of color filter which can be used. Color information is recovered through electronic demultiplexing.
Of the solid-state color cameras available, the single-chip camera, which captures images by single exposure, offers the least system complexity and lowest cost. It suffers, however, in terms of image quality when compared to other solid-state cameras having sensor chips of similar pixel density. Only one component color is captured per pixel location instead of three. This lack of information lowers the effective resolution of the sensor and increases the number of artifacts introduced through sampling. Of course, an improvement in effective resolution can be realized, and artifacts minimized, by increasing the pixel density, i.e., information content per unit area, of the chip. Constraints imposed by current technology, however, limit the extent to which this can be achieved. New technology must be developed to apply and pattern discrete color filters on pixels smaller than those available currently in state-of-the-art, color image sensors, ca. 5 .mu.m.times.5 .mu.m.
In general, the color filters used in single-chip cameras are organic-based, patternable layers which incorporate dyes to provide color separation. See, for example, U.S. Pat. Nos. 4,315,978 (Hartman et al) and 4,808,510 (Pace et al). In comparison to multilayer, dielectric color filters, the organic dye filters are less durable (i.e., softer); exhibit lower transmittance, especially in the blue portion of the visible spectrum; have lower thermal stability; and bleach or degrade with time. Also, their spectral characteristics are fixed and cannot be taylored or optimized for specific sensors or applications.
Multilayer, dielectric color filters are, therefore, the preferred type filter for application in single-chip color cameras. They are, however, increasingly difficult to form in small, pixel-sized elements as pixel size decreases. The conventional method of forming multilayer, dielectric color filters for sensors in single-chip cameras involves the use of lift-off processing. With this approach a separate lift-off process is used for each component color to pattern filters deposited by evaporation directly onto sensors or onto glass for subsequent attachment to sensors. As shown schematically in the cross-section views of FIG. 1, the lift-off pattern is formed first in a layer of photoresist 110 on substrate 120 (FIG. 1A). The multilayer, dielectric color filter 130 is next deposited over this lift-off pattern (FIG. 1B). Finally, the photoresist 110 is dissolved away with a suitable photoresist remover to `lift-off` all unwanted material (FIG. 1C). While this technique is useful to pattern a wide variety of materials, it offers two main disadvantages when used for patterning multilayer, dielectric color filters:
1. Shadowing, which occurs during deposition of the filters, produces filters with rounded edges. Rounded edges reduce the useable width of the filters by an amount which depends on the initial height of the lift-off pattern and the geometry of the deposition system. A width reduction of 1 .mu.m (0.5 .mu.m from each edge) or more is not uncommon. PA1 2. In a lift-off process, the temperature during deposition of filters is limited to below the temperature at which the photoresist begins to deform or degrade, i.e., typically less than 150.degree. C. Often, this limitation prevents the deposition of filters which are fully dense and non-absorbing. PA1 1. The thickness of, and number of layers in, a multilayer, dielectric color filter are determined by the materials used and the spectral characteristics required by the application. In Gale et al and Curtis et al the spectral characteristics required by the application allowed use of SiO.sub.2 /TiO.sub.2 filters of thickness less than 0.75 .mu.m with 10 or fewer layers. Each filter was patterned using photoresist as the dry etch mask. In general practice, however, multilayer, dielectric filters of thickness greater than 0.75 .mu.m are often required. For example, filters used in electronic cameras which scan photographic films may easily approach or exceed 3 .mu.m in thickness. Patterning of filters in this regime of thickness can require, depending on the selectivity of the dry etch (i.e., etch rate of a material relative to that of the mask), photoresist masks of thickness greater than that obtained (ca. 0.3-2 .mu.m) in a single spin-coat application. Multiple coatings may be required. As the thickness of the photoresist mask increases, the resolution of a dry etch and the accuracy to which edges can be defined, both decrease. Photoresist, therefore, is not the ideal mask for use in achieving optimum edge definition for general patterning of thick, multilayer, dielectric color filters. PA1 2. In any dry etch (reactive sputter etch, reactive ion etch (RIE), ion beam etch, etc.), batch or single wafer process, variations in etch depth occur over the active area of the etch system. For maximum throughput during an etch process the maximum amount of active area must be utilized. When attempting to stop an etch in a specific, etchable layer of a multilayer, dielectric filter (when the active area of the etch system is covered by such filters) variations in etch depth occur which cause variations in the performance of resultant filters. Some means to uniformly stop the etch, i.e., provide an etch-stop, is required. Gale et al demonstrated two approaches to address this issue utilizing the bottom layer of the top filter as the etch-stop. (i) The dry etch was stopped in the bottom layer when composed of SiO.sub.2 and the remaining SiO.sub.2 was removed by a wet chemical etch. The wet chemical etch was chosen such that it did not attack the top layer (i.e., TiO.sub.2) of the bottom filter. With the bottom layer of the top filter composed of SiO.sub.2 the dry etch was carefully monitored (by in-situ optical monitoring) and stopped in this layer. The dry etch was not highly selective between this layer and the underlying layer of TiO.sub.2 so, without care, the TiO.sub.2 would etch upon overshoot. Also the wet chemical etch, when used to remove the remainder of the SiO.sub.2 layer, would, due to its isotropic nature, attack all SiO.sub.2 layers in the top filter inward from the edges. This reduced useable filter area. (ii) To reduce the problems associated with SiO.sub.2 as the bottom layer of the top filter the SiO.sub.2 was replaced by SnO.sub.2. No mention was made as to the selectivity of this material with respect to TiO.sub.2 in the dry etch and the wet etch used to remove the SnO.sub.2, although not reactant with SiO.sub.2 or TiO.sub.2, required careful control to prevent undercut of the the top filter (inward from the edges). PA1 a) depositing a multilayer, dielectric color filter on a substrate having top, bottom and multiple intermediate layers; PA1 b) applying a patternable mask onto the top layer to provide selected openings through the mask; PA1 c) removing the top layer through the selected openings in the patterned mask, the patterned mask and the multiple intermediate layers of the filter being resistant to the process used for removing the top layer, to provide openings to the multiple intermediate layers of the filter; and PA1 d) removing, through the openings in the top layer, the multiple intermediate layers of the filter, down to the bottom layer, the top layer and bottom layer being resistant to the process used for removing the intermediate layers.
Another method for patterning multilayer, dielectric color filters for sensors in single-chip cameras, which avoids the disadvantages described above for the lift-off process, involves the use of an anisotropic etch. An anisotropic etch is an etch which attacks the filters faster in the direction normal to their surface than in a direction parallel to their surface. In this method, as shown schematically in the cross-section views of FIG. 2, a multilayer, dielectric color filter 210 is deposited on substrate 220 (FIG. 2A), covered by a patterned mask 230 (FIG. 2B) and etched (FIG. 2C). Finally, the patterned mask 230 is removed (FIG. 2D). This method permits the formation of filters with near vertical sidewalls, i.e., with little edge rounding and more useable surface area. It also separates the deposition from the patterning process permitting deposition of filters at higher temperatures with improved material properties.
Successful demonstrations of this method from a single research/development effort are described by Gale et al U.S. Pat. No. 4,534,620! and Curtis et al J. Vac. Sci. Technol. A 4(1), 70 (1986)!. In these demonstrations two multilayer, dielectric color filters (yellow and cyan) were utilized; one deposited on top of the other on glass substrates. The filters were patterned using dry etch processing to form regions comprised of green (yellow plus cyan), yellow or cyan, and white. Gale et al describe the top layer of the bottom filter as composed of a material (e.g., TiO.sub.2) which is inert to a given wet etchant (e.g., HF or concentrated HCl with small amounts of Zn powder added to produce nascent H.sub.2) and the bottom layer of the top filter as composed of a material (e.g., SiO.sub.2 or SnO.sub.2) which is etched by the given wet etchant (SiO.sub.2 by HF, SnO.sub.2 by concentrated HCl with small amounts of Zn powder). The remaining portions of the filters were composed of alternate layers of TiO.sub.2 and SiO.sub.2. Using photoresist as the mask an anisotropic, dry etch process (i.e., reactive sputter etching in CHClF.sub.2) was used to pattern the filters by removal of the top (yellow or cyan) filter and both (cyan and yellow) filters from selected regions. During patterning of the top filter the anisotropic dry etch was stopped inside the bottom layer (which was composed of either SiO.sub.2 or SnO.sub.2). The remainder of the bottom layer was removed by wet etching to expose the top layer of the bottom filter. (The top layer of the bottom filter being inert to the wet etch, was exposed but not attacked.) The dry etch through the bottom filter was stopped once inside the glass substrate.
An alternative demonstration of the above method is described in both Curtis et al and Gale et al wherein the remainder of the bottom layer of the top filter (i.e., SiO.sub.2), left behind by the dry etch, was left intact and not removed by wet etch. An epoxy overcoat with refractive index matching that of the SiO.sub.2 layer was applied over the patterned filter.
The patterning achieved through the top (yellow or cyan) and bottom (cyan or yellow) filters was claimed to provide an edge definition (separation between adjacent filters due to sidewall slope) for all filters of better than 1 .mu.m. The thickness of the top filter (10 layers) with a 0.1450 .mu.m thick, SiO.sub.2, bottom layer was 0.5734 .mu.m. The thickness of the corresponding bottom filter (9 layers) was 0.7208 .mu.m. The thickness of the top filter (9 layers) with a 0.1236 .mu.m thick, SnO.sub.2, bottom layer was 0.7412 .mu.m. The thickness of the corresponding bottom filter (9 layers) was 0.4360 .mu.m. The depth of the dry etch was controlled by in-situ reflectance monitoring.
Unfortunately, a number of limitations arise when attempts are made to utilize the methods described by Gale et al and Curtis et al to pattern multilayer, dielectric filters of random color in a general manufacturing process:
The use of isotropic wet etchants to remove the remainder of the bottom layer of the top filter without attacking the top layer of the bottom filter is not an optimum solution to achieving maximum useable filter area and edge definition. There are no means to prevent isotropic wet etches from undercutting the top filter inward from the edges. Both Gale et al and Curtis et al appear to have realized this as they describe an alternate method whereby use of a wet etch to remove the remainder of the bottom layer of the top filter is avoided. In this method the remainder of the bottom layer is not removed but, instead, left intact and covered by a polymer layer of similar refractive index. This method, although a reasonable alternative to the use of a wet chemical etch, is not universally applicable. It cannot, for example, be applied to the patterning of single filters when additional filters are subsequently deposited into the patterned openings.